Two common problems in treatments which involve drugs are drug-toxicity, which debilitates patients, and drug-resistance, which requires more drugs and thus amplifies the problem of drug-toxicity, often resulting in death. One way to solve the problem of drug-toxicity is to deliver drugs so they are targeted only to the diseased cells. Many researchers are working to develop antibodies to deliver drugs, and this approach holds promise, but antibodies are not without problems. For example, they often cross-react with normal tissues, and they can damage blood vessels (e.g., vascular leak syndrome) and cause dangerous allergic reactions (e.g. anaphylaxis).
The treatment of malignant cells by the delivery of drugs, including drugs that are toxic to such cells, is not new. U.S. Pat. Nos. 4,886,780; 4,895,714; 5,000,935; and 5,108,987 to Faulk and U.S. Pat. No. 4,590,001 to Stjernholm et. al., describe cytotoxic or radioimaging materials conjugated to proteins, mainly to transferrin, as treatments for cancerous cells or for imaging cancerous cells. These publications disclose useful methods for making and using such materials.
It is known that stressed cells, such as, for example, human cells hosting a viral infection and human cells invaded by cancer, call for an increased delivery of nutrients, such as iron, by presenting an increased number of receptors for nutrient carriers, such as transferrin in the case of iron. The increase in receptors for nutrient carriers in stressed cells is known to be relatively constant and orders of magnitude greater in number than in unstressed cells, which are known to show receptors intermittently and in relatively smaller numbers. The publications listed above, and others, disclose taking advantage of the increased number of receptors, especially for transferrin, presented by cancer containing cells to deliver imaging materials or drugs or both to the stressed cell.
No single study has asked if all stressed cells have up-regulated transferrin receptors, or if all normal cells have down-regulated transferrin receptors, but data from many quarters suggest that all normal cells have down-regulated transferrin receptors. For example, immature erythrocytes (i.e., normoblasts and reticulocytes) have transferrin receptors on their surfaces, but mature erythrocytes do not (Lesley J, Hyman R, Schulte R and Trotter J. Expression of transferrin receptor on murine hematopoietic progenitors. Cell Immunol 1984; 83: 14-25). Circulating monocytes also do not have up-regulated transferrin receptors (Testa U, Pelosi E and Peschle C. The transferrin receptor. Crit Rev Oncogen 1993; 4: 241-276), and macrophages, including Kupffer cells, acquire most of their iron by a transferrin-independent method of erythrophagocytosis (Bothwell T A, Charlton R W, Cook J D and Finch C A. Iron Metabolism in Man, Blackwell Scientific, Oxford, 1979). In fact, in vivo studies indicate that virtually no iron enters the reticuloendothelial system from plasma transferrin (for review, see Ponka P and Lok C N. The transferrin receptor: role in health and disease. Int J Biochem Cell Biol 1999; 31: 1111-1137.). Macrophage transferrin receptors are down-regulated by cytokines such as gamma interferon (Hamilton T A, Gray P W and Adams D O. Expression of the transferrin receptor on murine peritoneal macrophages is modulated by in vitro treatment with interferon gamma. Cell Immunol 1984; 89: 478-488.), presumably as a mechanism of iron-restriction to kill intracellular parasites (Byrd T F and Horowitz M A. Interferon gamma-activated human monocytes downregulate transferrin receptors and inhibits the intracellular multiplication of Legionella. pneumophila by limiting the availability of iron. J Clin Invest 1989; 83: 1457-1465.).
In resting lymphocytes, not only are transferrin receptors down-regulated, but the gene for the transferrin receptor is not measurable (Kronke M, Leonard W, Depper J M and Greene W C. Sequential expression of genes involved in human T lymphocyte growth and differentiation. J Exp Med 1985; 161: 1593-1598). In contrast, stimulated lymphocytes up-regulate transferrin receptors in late G1 (Galbraith R M and Galbraith G M. Expression of transferrin receptors on mitogen-stimulated human peripheral blood lymphocytes: relation to cellular activation and related metabolic events. Immunology 1983; 133: 703-710). Receptor expression occurs subsequent to expression of the c-myc proto-oncogene and following up-regulation of IL-2 receptor (Neckers L M and Cossman J. Transferrin receptor induction in mitogen-stimulated human T lymphocytes is required for DNA synthesis and cell division and is regulated by interleukin 2. Proc Nat Acad Sci USA 1983; 80: 3494-3498.), and is accompanied by a measurable increase in iron-regulatory protein binding activity (Testa U, Kuhn L, Petrini M, Quaranta M T, Pelosi E and Peschle C. Differential regulation of iron regulatory element-binding protein(s) in cell extracts of activated lymphocytes versus monocytes-macrophages. J Biol Chem 1991; 266: 3925-3930), which stabilizes transferrin receptor mRNA (Seiser C, Texieira S and Kuhn L C. Interleukin-2-dependent transcriptional and post-transcriptional regulation of transferrin receptor mRNA. J Biol Chem 1993; 268: 13,074-13,080.). This is true for both T and B lymphocytes (Neckers L M, Yenokida G and James S P. The role of the transferrin receptor in human B lymphocyte activation. J Immunol 1984; 133: 2437-2441), and is an IL-2-dependent response (Neckers L M and Trepel J B. Transferrin receptor expression and the control of cell growth. Cancer Invest 1986; 4: 461-470).
The best understood material mentioned in the above-listed publications is a conjugate of transferrin and doxorubicin, a well known and effective cytotoxic molecule. Although effective against cancers, doxorubicin has a maximum lifetime dosage for humans due to its cumulative cardiotoxicity. The conjugate has been shown to be effective in surprisingly low doses in killing a variety of types of cancers, including drug-resistant cancers in humans.
Well known and presently used methods for treating serious viral infections, such as infections by the human immunodeficiency virus (HIV) include blocking cell receptors that the virus uses to enter the cell, interfering with fusion mechanisms, and interfering with cell enzymes hijacked by the virus such as proteases and reverse transcriptases. Such methods and the drugs used in the methods, although effective in prolonging the life of seriously ill patients, have not resulted in wholesale cures. The materials themselves are well known to be prohibitively expensive for use in many parts of the world. Often, treatment with such drugs burdens a patient with complex dosing schemes and presents unpleasant side effects.
There is a need for materials for use in the treatment of high profile conditions such as AIDS, caused by HIV infections, that provide more effective results at a lower cost and fewer side effects for patients. There is also a need for materials for use in treating cells infected with a variety of other viral infections that burden societies, such as cytomegalovirus, adenoviruses, hepatitis viruses, herpes simplex viruses, and the like. There is also a need for drugs that kill such viruses and a variety of cancers without the use of cytotoxic materials even in small amounts.